Directional drilling in the build up section at North Oman Field in the Sultanate of Oman is challenging because of highly intercalated and unstable formations. The formation consists primarily of soft to medium hard limestones, dolomites inter- bedded with anhydrite, and a thick layer of unstable shale that forms the cap-rock of the targeted reservoir. The field development plan (FDP) specifies drilling long horizontal wells with two casing strings and a liner, and requires the reservoir section to be placed within the top meter of reservoir rock. Consequently, it is critical to land inside the top meter of the reservoir. The lack of directional control throughout the soft intervals, high dip angles in some of the formations, and the short vertical section from surface to landing point requiring BUR of 5 to 6 degrees/30mts created complications in drilling the build section. With these elements, optimum directional tools steerability is necessary to achieve the well objectives.

To overcome these challenges, the operator and drill Bit Company developed an optimization process to design a polycrystalline diamond compact (PDC) bit to use with point-the-bit rotary steerable systems (RSS). The optimization process included increasing the bit steerability and stability to increase the rate of penetration (ROP). This challenge required the development of a new PDC technology with optimized drilling practices and a reliable drive system. Specific bit design algorithms were incorporated with a new bit gage configuration, and drilling simulation software was used to optimize bit cutting structure design in the directional drilling environment. Significant bit design improvements were achieved. Going through field testing, drilling parameter evaluation and optimization, an 8½-in PDC bit design was created that established benchmark performances in the North Oman Field field.

The successful development of the PDC bit with the RSS system enabled record bit runs in the build sections, which were drilled in accordance with the FDP with significant ROP improvements compared to conventional motor. This paper reviews the bit design and optimization process that helped to improve the drilling performance and hole geometry, reduce rig days, and enable early delivery of the wells in the challenging Sultanate of Oman drilling environment.

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